Printers typified by ink jet printers and electrophotographic printers generally generate print data by receiving RGB color signals as input and converting the signals to signals indicating the color components of color materials (cyan [C], magenta [M], yellow [Y], and black [K]) equipped in the printers. This conversion is called color separation processing. One mainstream method employs a combination of a three-dimensional look-up table (LUT) and interpolation operation.
The three-dimensional LUT generally stores only values on grid points in an input color space, which are thinned out to about 17×17×17. Values between the grid points are calculated by interpolation operation. The interpolation operation can cause an error, leading to a decrease in the accuracy of color reproduction between the grid points.
PTL 1 and PTL 2 disclose techniques for reducing a decrease in accuracy due to interpolation operation by increasing linearity in interpolation operation with a three-dimensional LUT by using a one-dimensional LUT for performing nonlinear conversion before using the three-dimensional LUT. These techniques are recognized as techniques for appropriately linearly converting input signals to improve the accuracy of interpolation operation using a multidimensional LUT. Specifically, PTL 1 discloses a technique for nonlinear conversion using a one-dimensional LUT corresponding to one kind of input signal. PTL 2 discloses a technique for nonlinear conversion using different one-dimensional LUTs for different kinds of values input to a three-dimensional LUT, that is, different channels of red (R), green (G), and blue (B).
However, the nonlinear conversion for enhancing the linearity of interpolation operation using a multi-dimensional LUT depends on not only the kinds of values (for example, RGB) input to the multi-dimensional LUT but also the kinds of values (for example, CMYK) output from the multi-dimensional LUT. For this reason, the configuration in which only nonlinear conversion LUTs (one-dimensional LUTs) corresponding to input signals is provided needs further improvement to reduce a decrease in the accuracy of color reproduction due to an interpolation error. In particular, an ink-jet printer equipped with pale-color ink, which is lower in density than dark ink, significantly differs in nonlinearity between the pale ink, which is frequently used for a light region, and the dark ink, which is frequently used for a dark region.